1. Field of the Invention
This invention relates to an accurate low-cost load cell for use in mass-produced weighing devices--such as scales for weighing persons, packages, mail or kitchen items--and to scales and other devices incorporating such a load cell, particularly, but not necessarily, limited to scales in which either
(a) the design of the scale calls for the use of a plurality of load cells - e.g. four - requiring the maximum reduction of cost per load cell; or PA1 (b) where the design of the scale calls for a low-profile load cell, e.g. less than 0.125" in thickness; or both. PA1 (a) The load cells must be insensitive to transverse forces acting on them; PA1 (b) The load cells in a given scale must be of near-equal sensitivity to ensure that the reading remains within the allowed margin of error when the load bearing plate is subject to eccentric loads; PA1 (c) The mechanical elements of the load cells should not require significant machining or manual labor in assembly; PA1 (d) The load cells should not require individual trimming once they are mounted in a scale; PA1 (e) The load cells should be protectible against impact and overload; PA1 (f) Particularly when the scale plate or plates to which the load cells are to be attached is of limited rigidity, the load cells should not create strong forces or bending moments on the plates; PA1 (g) It should be possible to bond the electronic strain sensors--be they strain gages, thick film resistors or semi-conductors--to the mechanical element of the load cell in a simple fashion, e.g. directly on to one side of a flat exposed surface. PA1 (a) Manual calibration of each load cell after it is mounted in a scale, changing the dimensions of the gaged beam by trimming, filing or grinding; or PA1 (b) Pre-programming a micro-processor in each scale with the initial sensitivity of each load cell. PA1 a. a one-piece, flat, mechanically-deformable metal part, comprising PA1 the flexure beam bends under load as a double cantilever beam into a symmetrical S-shape, producing stresses of equal and opposite sign at points equidistant from its mid-point along its main axis; and PA1 b. At least one pair of strain sensors mounted on at least one of the flat surfaces of the flexure beam, with their centers substantially along its main axis and substantially equi-distant from its mid-point, each pair of sensors forming half a Wheatstone Bridge and producing signals of nominally equal and opposite values when the load cell is under load. PA1 a. A plurality of said load cells; PA1 b. A rigid bottom plate to which the mounting elements of the load cells are fastened on a plurality of flat protrusions; PA1 c. At least two impact-resistant pads attached to the top surfaces of the leading edges of the U-shaped loading element and centered at points opposite each other across the mid-point of the flexure beam of each load cell; PA1 d. A rigid top load bearing plate resting on and affixed to the impact resistant pads of the plurality of load cells; PA1 e. Electronic means and power supply means for summing the output of the plurality of load cells, converting it to a digital signal, translating the digital signal into pounds or kilograms and displaying the resulting weight for viewing. PA1 a. A rigid load bearing composite plate assembly made of top and bottom thin plates rigidly bonded together, and containing a plurality of cavities between the two plates; PA1 b. A plurality of the said load cells mounted inside the cavities of the load bearing plate assembly, the U-shaped mounting elements of the respective load cells being rigidly anchored with spacers to the top and bottom plates so that the load cells float inside the respective cavities; PA1 c. Shallow feet connected to the bottom of each U-shaped loading element at points opposite each other across the mid-point of the respective flexure beam through holes in the bottom plate, each foot comprising: PA1 d. Electronic means and power supply means for summing the output of the plurality of load cells, converting it to a digital signal, translating the digital signal into pounds or kilograms and displaying the resulting weight for viewing.
2. Description of the Prior Art
Most electronic scale in wide use, and particularly in consumer use for personal weight measurement, use a set of levers which transmit the load on a load bearing plate to a single load cell. The load cell is usually composed of two elements: a mechanically-deformable element acting as a force transducer, and an electronic strain sensor which transforms the mechanical deformation into an electrical signal proportional to the load on the plate.
The lever mechanisms impose minimum thickness requirements on such scales, and limit their accuracy.
There is a different and more accurate principle for constructing scales--particularly scales with a low-profile design--which does not require any levers, and which has been in industrial and commercial use for some time. It involves placing a rigid load bearing plate on a plurality of load cells, and summing up the electrical signals from these load cells to obtain an accurate measure of the total load on the plate. Although several patents have been issued for portable electronic scales for consumer and commercial use embodying this principle--e.g., U.S. Pat. No. 4,335,692 issued to Osterlich, U.S. Pat. No. 4,394,079 issued to Brendel, U.S. Pat. No. 4,411,327 issued to Lockery, U.S. Pat. No. 4,739,848 issued to Tulloch, and U.S. Pat. No. 4,800,973 issued to the present inventor--none of these inventions has come into wide commercial use, in part because of the prohibitive cost of producing, for each scale, four accurate load cells with near-equal sensitivity.
The basic requirements for accurate, mass produced load cells for use in such embodiments are:
The prior art devices do not provide simple means of equalizing the sensitivity of individual load cells. Unequal sensitivity will usually result in a considerable reduction in the repeatability of measurements, especially when loads are placed in different locations on the load bearing platform. Correcting this flaw usually requires one of two known procedures:
Both procedures are cumbersome and expensive operations, usually unsuitable for consumer-oriented manufacturing.
Tulloch, Osterlich and Ryckman do not disclose means for retaining accuracy under eccentric loads, beyond the use of knife-edge mechanisms for ensuring that the loads on the load cells remain in the same general area. These knife-edge arrangements are known to be imprecise, and the knife-edges tend to lose their sharpness over time. The low-profile scale described in my prior patent, U.S. Pat. No. 4,800,973, also uses load cells which cannot retain their accuracy when subject to eccentric loads.
Brendel and Lockery use a well-known double-cantilever arrangement for retaining precision under eccentric load conditions by locating two strain gages on each flexure beam--one exposed to tension and one to compression of equal magnitude--so that additional moments created by transverse forces are cancelled.
The principle embodied in double-cantilever arrangements is described in detail in U.S. Pat. No. 4,565,555 issued to Sarrazin, for example, and forms part of the disclosure in U.S. Pat. No. 4,020,686 issued to Brendel: The signal generated by the two strain sensors bonded to the flexure beam is proportional to the sum of the bending moments on the beam at the center points of the gages. Since the sensors are located on the main axis of the middle beam, equi-distant from its mid-point, a force pressing on the leading edges of the U-shaped loading element is proportional to the product of the force and the distance between the sensors. Since the distance between the sensors is fixed, the signal will be proportional to the force even if the force is not exactly at the center of the flexure beam.
Each load cell forms at least half a Wheatstone Bridge, and four load cells, for example, can form a complete Wheatstone Bridge with two strain sensors on each arm of the bridge. For greater accuracy, two sensors can be bonded to both the top and the bottom of the flexure beam to form a complete Wheatstone Bridge for each load cell.
The load cells described in the embodiments disclosed by Lockery and Brendel, however, are not suitable for mass production. They contain a plurality of parts and require considerable machining, grinding, tapping, screwing, assembly and post-mounting trimming operations which increase their costs of production significantly.
The load cells described by Brendel and Lockery also create significant moments on the plates to which they are attached. This requires the plates to be able to resist these moments.
Such arrangements are not suitable for lighter-weight scales with weaker plates, or for very thin plates.
One or more of the limitations discussed above: inability to cancel transverse forces; difficulty in equalizing load-cell sensitivity; a multiplicity of machined parts requiring assembly; absence of impact and overload protection; and the creation of significant bending moments on the plates to which the load cells are attached have been found in all known embodiments for scales using a plurality of load cells, making it difficult to produce such scales for consumer use which would be both affordable and accurate, and making it particularly difficult to produce thin and light-weight scales which are truly portable.